Delhi's air quality is fluctuating between the very poor and severe categories.

There has been no really significant change of DELHI'S AQI, rather it seems to just deteriorating.

Recent findings from a joint project by the Delhi government and IIT-Kanpur found that vehicular emissions accounted for about 38% of the capital's air pollution on Wednesday. This is projected to rise to 40% on Thursday, as reported by PTI.

Secondary inorganic aerosols -- particles such as sulfate and nitrate that are formed in the atmosphere due to the interaction of gases and particulate pollutants from sources like power plants, refineries, and vehicles -- is the second major contributor to Delhi's foul air, accounting for 30 to 35% of the air pollution in the city over the last few days.

Calm winds and low temperatures are allowing accumulation of pollutants and relief is unlikely over the next few days, an official at the India Meteorological Department (IMD) said.

Delhi's Air Quality Index stood at 393 at 9 am on Thursday. Its 24-hour average AQI, recorded at 4 pm every day, stood at 401 on Wednesday. It was 397 on Tuesday. It was 358 on Monday and 218 on Sunday, 220 on Saturday, 279 on Friday and 437 on Thursday.

Neighbouring Ghaziabad (358), Gurugram (325), Greater Noida (343), Noida (337) and Faridabad (409) also recorded very poor to severe air quality.

Delhi's air quality dropped over the last few days despite the state government implementing stringent measures, including a ban on construction work and the entry of diesel-guzzling trucks into the city, to control pollution.

According to IQAir, a Swiss company that specialises in air quality monitoring, Delhi was the most polluted city in the world on Wednesday, followed by Lahore and Mumbai.

A system developed by the Pune-based Indian Institute of Tropical Meteorology to identify the contribution of different pollution sources showed stubble-burning accounted for 23% of the air pollution in the capital on Wednesday and 11% on Thursday. It is likely to come down to four% on Friday.